I'd add I think this occurs fairly often when some of the pre-PR days are under 'implied status', eg someone on a work or study permit who applies (not long) before expiry, so they're (technically) allowed to remain and then IRCC takes substantial time to approve, so that time in-between is not directly recorded and/or a grey area.
[Or put differently there's a period in-between where one visa expired and the new one is issued at a later date and the gap in-between is not recorded/confirmed.]
I'm not making a judgment on how this should be or is treated most often or what recourse an applicant has - just that it seems to come up.